1. Field of the Invention
The invention relates to a method of fabricating a light emitting device, and a light emitting device; in particular to a method of fabricating a light emitting device, and a light emitting device, which is suitable for the light emitting device performed the extraction of emitted light from an n-type semiconductor layer side.
2. Description of the Related Art
Light emitting devices having the light emitting layer portion thereof composed of (AlxGa1−x)yIn1−yP (where, 0≦x≦1 and 0≦y≦1, referred to AlGaInP alloy, or more simply AlGaInP, hereinafter) alloy can achieve a high luminance by adopting a double heterostructure in which a thin AlGaInP active layer is sandwiched between an n-type AlGaInP cladding layer and a p-type AlGaInP cladding layer which have a larger band gap. As for these devices, either the n-type AlGaInP cladding layer side or the p-extracting the emitted light obtained at the light emitting layer portion. It is to be noted that the object of the invention is the light emitting device which is extracted the light emitted at the light emitting layer portion from an n-type semiconductor layer side. According to this note, the emitted light is extracted from the n-type AlGaInP cladding layer side.
Referring now to AlGaInP light emitting device, a p-type GaAs buffer layer, a p-type AlGaInP cladding layer, an AlGaInP active layer, and an n-type AlGaInP cladding layer are stacked in this order on a p-type GaAs substrate, to thereby form a light emitting layer portion having a double heterostructure. Current supply to the light emitting layer portion is generally accomplished via a metal electrode formed on the surface of the device. Since the metal electrode functions as a light interceptor, it is typically formed so as to cover only the center portion of the main surface of the light emitting layer portion, so that the light can be extracted from the peripheral area having no electrode formed thereon.
In this case, reduction in the area of the metal electrode is advantageous in ensuring a larger light leakage area formed around the electrode, and in improving the light extraction efficiency. Although a number of efforts have been made to increase the amount of extractable light by effectively spreading current within the device based on various designs of the electrode shape, any of them could not exempt from increase in the electrode area, and fell in a dilemma that consequent reduction in the light leakage area undesirably limits the amount of light extraction. Another problem resides in that carrier concentration of the dopant, that is conductivity, is generally suppressed at a slightly lower level in order to optimize light emitting recombination of the carriers within the active layer, and this makes the current less likely to spread in the in-plane direction. This means undesirable concentration of current density into the area covered with the electrode, and reduction in substantial amount of extractable light from the light leakage area. A general method for solving this problem is to dispose a current spreading layer having a raised carrier concentration, and consequently having a low resistivity, between the cladding layer and electrode. On the other hand, another possible constitution relates to that a thick current spreading layer is disposed on the back side of the device so that the layer is used also as the substrate (while the current spreading layer in this case may be assumed as a conductive substrate, it is to be defined in the specification that the layer conceptually belongs to the current spreading layer in a broad sense). In most conventional cases, such current spreading layer has been formed by a metal organic vapor-phase epitaxy process (also occasionally referred to as a MOVPE process) together with the light emitting layer portion.
The current spreading layer provided in the light emitting device is generally designed so as to increase the thickness thereof to some extent in order to sufficiently spread the current in the in-plane direction, and typically formed with a larger thickness than the light emitting layer portion has. A MOVPE process, however, is slow in the layer growth rate, needs considerably long time for growing the current spreading layer to a sufficient thickness, and thus raises problems in degraded production efficiency and increased costs. Organo-metallic compounds used as Group III element sources in a MOVPE process are generally expensive. Moreover, it is necessary in a MOVPE process to use Group V element sources (e.g., AsH3, PH3) in a great excess (10 to several hundred times) of Group III element sources in order to improve the crystallinity, and this raises another disadvantage from the viewpoint of costs.
The current spreading layers grown by a MOVPE process are likely to contain residual H (hydrogen) and C (carbon) derived from the organo-metallic molecules. For the case where the current spreading layer is designed to have a conductivity type of n-type by doping with Si (Silicon), S (Sulfur), Se (Selenium) or Te (Tellurium), the residual C contributes as p-type dopant, so that a relatively large amount of Si (Silicon), S (Sulfur), Se (Selenium) or Te(Tellurium) of n-type dopant must be added in order to ensure a sufficient conductivity required for the current spreading layer. Addition of such the n-type dopant in a large amount will, however, raise the problems below.
The light emitting devices lower their luminance as the current supply thereto is prolonged. Assuming now that the emission luminance measured immediately after the start of current supply to the device at a constant current is defined as the initial luminance, and the emission luminance which decreases with the elapse of cumulative current supply time is traced. In this case, a time required for the emission luminance to reach a predetermined limit luminance, or a ratio of emission luminance after the elapse of evaluation current supply time with respect to the initial luminance (referred to as “device life”, hereinafter) under a constant evaluation current supply time (e.g., 1,000 hours) can be used as a kind of index for evaluating the device life. Excessive increase in the n-type dopant in the current spreading layer, in particular in a portion adjacent to the light emitting layer portion, tends to accelerate the degradation of the device life.
It is therefore a first subject of the invention to provide a method of fabricating a light emitting device capable of forming the current spreading layer to have conductivity type of n-type in an efficient manner. A second subject resides in providing a light emitting device having an improved device life even if n-type current spreading layer is used.